A plant for production of gasoline, liquid hydrocarbons such as diesel, or methanol from for example natural gas typically comprises of the following main process units: (a) air separation, (b) synthesis gas preparation via autothermal reforming (ATR), (c) the actual synthesis of e.g. diesel, (d) upgrading and/or separation. In the synthesis gas preparation section, hydrocarbon feedstock, normally natural gas is normally pre-reformed, and then passed through an autothermal reformer (ATR) to produce a synthesis gas. An oxygen containing stream is also added to the ATR. This synthesis gas is cooled, water is removed and the thus dehydrated synthesis gas is converted to a raw product. The raw product is then upgraded and/or separated from undesired by-products to provide the desired end product, such as diesel or gasoline.
Particularly, in a plant for production of diesel, Fischer-Tropsch (FT) synthesis is carried out for producing a mixture of hydrocarbons comprising for example wax and liquids as well as a range of lighter and gaseous hydrocarbons with hydrogen and carbon monoxide as reactants. In this case the upgrading section would normally comprise hydrocracking for production of the final product which is mainly diesel.
The FT synthesis often also produces an off-gas in the form of so-called tail gas comprising unreacted hydrogen and carbon monoxide and light hydrocarbons (typically with 5 or less carbon atoms) including olefins. The tail gas often also comprises carbon dioxide and other typically inert compounds such as nitrogen and argon. It is known to recycle tail gas to the ATR section to adjust the H2/CO-molar ratio in the synthesis gas to the desired value for FT synthesis which typically is around 2.
The prior art is silent about adding tail gas to a specific point of a separate plant for independent production of hydrogen.
For instance, US-A-2003/0134911 discloses a process integrating Fischer-Trospch synthesis and syngas production in which Fischer-Tropsch tail gas from the upgrading section is treated sequentially by hydrogenation and steam reforming. The thus reformed tail gas is added either to the synthesis gas entering the Fischer-Tropsch synthesis reactor, or to the pre-reformed gas prior to the autothermal reformer in the synthesis gas section of the plant.
US-A-2008/0312347 discloses a process for producing hydrocarbons via Fischer-Tropsch synthesis. Fischer-Tropsch tail gas from product separation stage is shifted and then passed through a carbon dioxide removal stage, a dehydration stage and a cryogenic stage. A part of the hydrogen from the cryogenic stage is mixed with methane also exiting such cryogenic stage. Then, sulfur is removed from the gas mixture and finally passed through a reforming stage where synthesis gas for downstream Fischer-Tropsch synthesis is produced.
WO-A-2014/057013 discloses a process in which tail gas from Fischer-Tropsch synthesis is treated by hydrogenation, shift and methanation before subjecting the resulting gas to autothermal reforming. The gas from the autothermal reforming, which is partially shifted, is sent to the Fischer-Tropsch section where it is mixed with synthesis gas produced by gasification.
US-A-2013/0082211 discloses a process in which a dry hydrocarbon feedstock, preferably tail gas from Fischer-Tropsch synthesis, is treated by the sequential stages of hydrogenation, water gas shift and reforming. The thus resulting synthesis gas is combined with a gaseous feedstock derived from a carbonaceous feedstock and fed to a Fischer-Tropsch plant for production of hydrocarbons.
US-A-2013/0065947 discloses in its FIG. 4 a process for diesel production in which naphtha and Fischer-Tropsch tail gas are added together with natural gas to the a pre-treatment unit upstream the steam methane reformer.
The pre-treatment unit consists of a hydrotreater, desulfurization unit and pre-reformer unit.
US-A-2013/0090393 discloses a process in which tail gas from Fischer-Tropsch synthesis is treated by the sequential stages of water gas shift, carbon dioxide-removal and hydrogen separation in a PSA-unit (Pressure Swing Adsorption) to produce a hydrogen stream used to upgrade the heavy fraction of the Fischer-Tropsch synthesis.